Superconductor quantum interference devices (SQUIDs) are sensitive devices especially suited for detecting small changes and spatial variation in magnetic fields. This feature of said devices finds wide application in medical research, magnetometers (for accurate measurement of magnetic susceptibility), and geophysical laboratories. SQUIDs can be of two kinds rf and dc. The former consists of a single superconductor-insulator superconductor junction in a loop. The latter consists of two similar such junctions in a loop.
Superconductivity was originally discovered by the Dutch scientist Heike Onnes in 1911 while he was studying the electrical properties of mercury at very low temperatures. In more recent times, Ogg (1946) studied superconductivity in ammonia solutions and proposed that superconductivity arose in these quenched metal-ammonia solutions because of mobile electron pairs. About 1973, it was determined that certain niobium metal alloys exhibited superconductivity when cooled to liquid helium (4 K.) temperatures. Later results in the 1970's raised this temperature as high as 23 K. (-250.degree. C.). Until recently, it was believed that superconductivity above this temperature was not possible. This belief was based on the theoretical work of Bardeen, Cooper and Schieffer (BCS theory-1946) which predicted such a limit. In December 1986, Bednorz and Mller announced the discovery (G. Bednorz and A. Mller, Z. Phys., B64 189 (1986)) of a new ceramic superconducting compound based on lanthanum, barium, and copper oxides, whose critical temperature for superconductivity was close to 35 K. By the following month, the critical temperature, T.sub.c, for the onset of superconductivity was raised to nearly 80 K. by C. W. Chu and coworkers (M. K. Wu, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58 908 (1987)). This was achieved by changing the composition to yttrium barium copper oxide, approximated by the formula: Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3 Since then, a number of families of superconducting ceramic oxides have been investigated, including:
Bismuth Strontium Calcium Copper Oxide:
Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y PA1 T.sub.c =114 K. PA1 Tl Ba.sub.2 Ca Cu.sub.2 O.sub.7 PA1 Tl Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 PA1 Tl Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.11 PA1 Tl Ba.sub.2 Ca.sub.4 Cu.sub.5 O.sub.13 PA1 T.sub.c =120 K. PA1 BaO--K.sub.2 O--Bi.sub.2 O.sub.3 PA1 T.sub.c =30 K.
Thallium Calcium (Barium) Copper Oxide:
There have been some scattered reports of superconductivity above 162 K., For instance, R. G. Kulkarui has reported superconducting oxides having an approximate composition 0.5CaO. 0.5ZnO. Fe.sub.2 O.sub.4, with critical temperatures in this range. Ogushi also reported superconductivity at room temperature in yet ill-defined niobium strontium lanthanum oxides. While these reports have yet to be confirmed independently by other researchers, it is reasonable to expect that superconductors with critical temperaures near to room temperature will soon be obtained.
Niobium-based superconducting alloy wires have long been been used for detecting small changes in magnetic field strength. In the prior art, superconductor quantum interference devices have typically been made of Nb-Sn or similar low temperature superconductors that operate at liquid H2 temperatures. With the discovery of high-T.sub.c superconductors and high-T.sub.c superconducting thin films, high-T.sub.c superconducting superconductor quantum interference devices also find application in non-destructive evaluation of materials, especially metals.
I have determined that the use of superconductor quantum interference devices to measure changes in magnetic field strength produces results much superior to any known heretofore, especially when said devices are used in conjunction with modulated high frequency exciting currents.